The present invention relates to a fiber optic cable having at least one optical fiber ribbon therein.
Conventional fiber optic cables may include optical fiber ribbons having coated glass optical fibers that are used to transmit voice, video, and data information. The optical ribbons typically include a UV-curable common matrix coating that surrounds the optical fibers. In a typical method of manufacturing an optical ribbon, the common matrix coating is extruded about a group of optical fibers that have been arranged in a planar array. The common matrix coating is then irradiated with a UV light source which cures it, thereby fixing the relative positions of the fibers in the planar array. Indoor and outdoor fiber optic cables may include one or more optical cable sub-units in the form of an optical ribbon surrounded by tensile strength members and a jacket.
U.S. Pat. No. 5,253,318 discloses examples of optical cable sub-units that include a layer of uniformly distributed PTFE-buffer material of essentially constant thickness, polyester insulation, braided aramid sheath or aramid cloth, and a fire retardant jacket. More particularly, the sub-unit has layers including: polymer coated optical fibers arranged parallel to each other; a layer of porous expanded PTFE, each side of which is coated by an adhesive; at least one layer of a polyester tape coated on at least its inner side with a polyester adhesive layer; a braided sheath of aramid fibers; a layer of tape-wrapped binder tape; and a flame retardant jacket. The various thermoplastic layers/adhesives of the foregoing sub-unit make it expensive and difficult to manufacture. Additionally, the optical fibers thereof may experience an unacceptable level of attenuation as a result of stress acting on the optical fibers during cable bending or temperature cycling.
Attenuation bears upon the performance of an optical cable. Attenuation indicates a degradation in optical performance due to a loss in power from one point to another along an optical fiber, and it may be caused by microbending or macrobending of the optical fiber. A delta attenuation is the change in attenuation that a modified optical fiber experiences as compared to a reference attenuation measurement of the optical fiber in a pre-modified state. Where an optical cable is bent, microbending and/or macrobending may occur in the optical fibers therein resulting in an unacceptably high delta attenuation.
In view of the foregoing, it is an object of the present invention to provide a fiber optic cable including: an optical ribbon having optical fibers in a common matrix coating, a jacket, and a buffer material between the common matrix coating and the jacket; the buffer material including at least one volume of buffer material defining a stress-cushioning zone between the optical ribbon and the jacket, the stress-cushioning zone being operative to substantially decouple the jacket from the ribbon in the region of the stress-cushioning zone thereby inhibiting the application of stress to the optical ribbon; and the buffer material including at least one volume of material at an intermediate buffer zone held substantially tight against an intermediate portion of the optical ribbon for inhibiting the twisting of the optical ribbon.
It is another object of the present invention to provide a fiber optic cable including: an optical ribbon having optical fibers in a common matrix coating, a jacket, and a buffer material between the common matrix coating and the jacket; volumes of buffer material defining stress-cushioning zones between the optical ribbon and the jacket for avoiding attenuation in the optical ribbon when the cable is stressed; volumes of buffer material defining intermediate buffer zones; and the buffer material being substantially non-uniformly distributed about the optical ribbon such that the volumes of the buffer material in the stress-cushioning zones is substantially different than volumes of the buffering material in the intermediate buffer zones.